As a substrate for forming a semiconductor device with a high speed and low power consumption, a substrate having a strained silicon layer has received a great deal of attention. When a layer (SiGe layer) made of silicon (Si) and germanium (Ge) is grown on a silicon substrate, and a single-crystal silicon layer is grown on the resultant structure, the silicon layer is strained. Thus, a strained silicon layer is obtained. This strain occurs because the lattice constant of the layer made of silicon and germanium is slightly larger than that of the single-crystal silicon layer.
An SOI substrate having a buried oxide film in a silicon substrate has also received attention and been put into practical use as a substrate for forming a semiconductor device with a high speed and low power consumption.
Also, a technique has been reported, in which a first SiGe layer is formed on a silicon substrate, a second SiGe layer having a higher Ge concentration than the first SiGe layer is formed, and a buried oxide film serving as an insulating layer is formed near the interface between the first and second SiGe layers by SIMOX (Separation by Ion iMplanted OXygen), thereby obtaining a thin SiGe layer with a high Ge concentration on the buried oxide film (“A Novel Fabrication Technique of Ultra-Thin and Relaxed SiGe Buffer Layers with High Ge Content for Sub-100 nm Strained Silicon-On-Insulator MOSFETs”, T. Tezuka et al., EXTENDED ABSTRACTS OF THE 2000 INTERNATIONAL CONFERENCE ON SOLID STATE DEVICES AND MATERIALS, Sendai, 2000, pp. 472–473; “Design of SiGe/Buried Oxide Layered Structure to Form Highly Strained Si Layer on Insulator for SOI MOSFETs”, N. Sugiyama et al., EXTENDED ABSTRACTS OF THE 2000 INTERNATIONAL CONFERENCE ON SOLID STATE DEVICES AND MATERIALS, Sendai, 2000, pp. 474–475).
As a characteristic feature of the technique by T. Tezuka et al. and N. Sugiyama et al., SIMOX is used to form a structure with an SiGe layer on an insulating layer. Hence, this technique latently has a technical disadvantage in SIMOX. In SIMOX, a large number of oxygen ions are implanted into a silicon substrate to form a buried oxide film in the silicon substrate. For this reason, in SIMOX, many crystal defects are formed in the silicon substrate, and it is therefore difficult to ensure quality enough to form a minority carrier device. In addition, the oxide film formed in the silicon substrate by SIMOX requires a higher quality. These points are taken into consideration. In the technique reported by T. Tezuka et al. and N. Sugiyama et al., a number of crystal defects (e.g., dislocation) are generated in the SiGe layer by the SIMOX process. Additionally, it is difficult to improve the quality of the buried oxide film. Hence, it is supposed to be difficult to make full use of the latent effects of the strained silicon and SOI structure.